Several metabolites formed by microorganisms have been discovered to act as competitors with vitamin B12 for the binding sites on the special proteins which aid in the transport of this health-important vitamin from the intestinal tract to the target organi in the body. These inhibiting metabolites may be useful in developing model systems for studying the mechanisms of action of vitamin B12 and and also as therapeutic agent which may be helpful in restricting the growth of tumor cells which have an abnormally high vitamin B12 requirement.